1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method for effectively transmitting a control signal in a wireless communication system.
2. Related Art
Next-generation mobile communication systems are being standardized for the purpose of efficient connection with wired and wireless communication networks and integrated service although communication systems of previous generations provide simple wireless communication services. As high-speed large-capacity communication systems capable of processing and transmitting information of various types such as video data and radio data as well as audio data are required, development of a technique capable of transmitting a large quantity of data, which corresponds to the capacity of a wired communication network, through a wireless communication network is needed. Accordingly, an appropriate error detection scheme which can minimize information loss and increase system transmission efficiency to improve system performance becomes an essential element.
Automatic Repeat request (ARQ) transmits positive acknowledgement (ACK) signal to a transmitter when a receiver properly receives data and transmits a negative acknowledgement (NACK) signal to the transmitter when the receiver does not properly receive the data. In Hybrid Automatic Repeat request (HARQ), an ACK/NACK signal transmitted by a data receiver is represented by a small number of bits, in general.
To improve data transmission efficiency in data processing, HARQ corresponding to a combination of ARQ and channel coding of a physical layer has been proposed. HARQ not only re-transmits data that is not received by a transmitter but also store data that is not received by a receiver. When the receiver receives re-transmitted data, the received data is added to the previously stored data to improve a performance gain.
Since the receiver uses additional feedback radio resources to feed back an ACK/NACK signal to the transmitter in HARQ, efficient use of limited feedback radio resources is very important.
Hereinafter, a downlink represents a communication link from a base station to a user equipment (UE) and an uplink represents a communication link from the UE to the base station. The downlink is also referred to as a forward link and the uplink is also referred to as a reverse link. A transmitter may be a part of the base station and a receiver may be a part of the UE in the downlink. The transmitter may be a part of the UE and the receiver may be a part of the base station in the uplink.
A method of discriminating radio resources used for downlink transmission from radio resources such as frequency, time and code domains used for uplink transmission is required. This method is referred to as duplex. The uplink and the downlink can be discriminated from each other in the frequency, time and code domains as does in a multiple access scheme for identifying different users. The duplex is classified into frequency division duplexing (FDD) which discriminates the uplink and the downlink from each other according to frequency and time division duplexing (TDD) which discriminates the uplink and the downlink from each other according to time.
The uplink is discriminated from the downlink in the frequency domain in FDD, and thus transmission of data between a base station and a UE can be continuously performed in the time domain in each link. While FDD is suitable for symmetric service such as voice communication because it symmetrically allocates frequencies having the same level to the uplink and the downlink, TDD is suitable for asymmetric service such as Internet service and thus researches on the TDD have been actively carried out recently.
TDD is suitable for the asymmetric service because it can allocate time having different lengths to the uplink and the downlink. Furthermore, uplink data and downlink data are transmitted and received in the same frequency band in TDD, and thus uplink and downlink channel states correspond to each other. Accordingly, TDD is suitable for array antenna technology because a channel state can be immediately estimated when a signal is received. TDD uses the entire frequency band as the uplink or the downlink, discriminates the uplink from the downlink in the time domain, uses the frequency band as the uplink for a predetermined time and uses the frequency band as the downlink for a predetermined time, and thus transmission and reception of data between a base station and a UE cannot be simultaneously performed.
When a base station transmits downlink data in a mobile communication system, a UE transmits ACK/NACK signals with respect to the downlink data to the uplink after a predetermined lapse of time. If the time used for downlink transmission is longer than the time used for uplink transmission, the number of ACK/NACK signals to be transmitted to the uplink may be restricted. That is, a single UE should transmit ACK/NACK signals by using a number of ACK/NACK resources smaller than N for received N downlink packets. Accordingly, there is a need for an ACK/NACK signal transmitting method capable of minimizing packet loss and maximizing recovery capability even when a number of ACK/NACK signals smaller than the number of downlink packets are fed back.